Balanced architecture for adhesive image media

ABSTRACT

The invention relates to an image element comprising at least one image layer, a base, a gelatin layer below said base and a pressure sensitive adhesive below said gelatin layer, wherein said base has a stiffness of less than 20 mN.

FIELD OF THE INVENTION

The invention relates to controlling the curl of imaging elementscontaining both gelatin and a pressure sensitive adhesive at lowrelative humidities and high temperatures through the use of a balancedarchitecture. In a preferred form it relates to the use of silver halidepressure sensitive label for the printing of text, graphics and imagesapplied to packaging material having good curl resistance at lowrelative humidities and high temperatures.

BACKGROUND OF THE INVENTION

It has been proposed in U.S. Pat. No. 4,507,166 to apply an adhesivecoated release sheet to the backside of exposed, developed photographicpaper prior to the cutting of the photographic paper into strips orsheets. While this method of creating adhesive backed photographs doesproduce an acceptable adhesive backed image it is inefficient andcostly. The photofinisher must purchase additional special equipment andan adhesive coated release sheet to apply the adhesive to the backsideof the photographic paper. It would be desirable if a photographic papercontained a repositionable adhesive that did not require thephotofinisher to purchase extra materials or equipment to provide anadhesive backed photograph. Further, adhesive systems post processapplied to photographic paper provides stiffness greater than 130millinewtons to the imaging layers which resist the curling forces ofthe gelatin binder used in photographic imaging layers.

Present digital repositionable images that are typically used forstickers and dry mounting of digital images are constructed using arepositioning adhesive with an adhesive liner applied to the backside ofthe imaging layer. The adhesive system is typically applied in themanufacturing process for digital image support and the adhesive isexposed by the consumer after the image has been formed in the digitalimaging layer. The most widely used technology for the formation of theimages is inkjet printing. While ink jet imaging does provide acceptableimage quality for some repositionable imaging applications, it suffersfrom a long dry time and at present cannot match the image quality ofsilver halide imaging systems. There remains a need for a high qualitysilver halide reflective receiver with a peelable and repositionableadhesive layer.

In the formation of color paper it is known that the base paper hasapplied thereto a layer of polymer, typically polyethylene. This layerserves to provide waterproofing to the paper, as well as providing asmooth surface on which the photosensitive layers are formed. While thepolyethylene does provide a waterproof layer to the base paper, the meltextruded polyethylene layer used in color paper has very littledimensional strength and as a result can not be used alone as a carrierof an image. It has been proposed in U.S. Pat. No. 5,244,861 to utilizebiaxially oriented polypropylene in receiver sheets for thermal dyetransfer. In U.S. Pat. No. 5,244,861 high strength biaxially orientedsheets are laminated to cellulose paper with low density polyethylene.While the biaxially oriented sheet in U.S. Pat. No. 5,244,861 is anefficient thermal dye transfer support, the biaxially oriented layercannot be stripped from the paper and reapplied to a different surface.

Adhesive layers are also utilized for adhering labels to consumerproducts. Pressure sensitive labels are applied to packages to buildbrand awareness, describe the contents of the package, convey a qualitymessage regarding the contents of a package and supply consumerinformation such as directions on product use, or an ingredient listingof the contents. Printing on the pressure sensitive label is typicallyprinted by using gravure printing or flexography is applied to thepackage. The three types of information applied to a pressure sensitivelabel are text, graphic and images. Some packages only require one typeof information while other packages require more than one type ofinformation.

Prior art labels that are applied to packages consist of a face stockmaterial, a pressure sensitive adhesive and a liner. The label substrateconsisting of the face stock, pressure sensitive adhesive and liner aretypically laminated and then printed utilizing a variety ofnon-photographic printing methods. After printing, the labels aregenerally protected by an over laminate material or a protectivecoating. The completed label consisting of a protection layer, printedinformation, base and pressure sensitive adhesive is applied to packagesutilizing high speed labeling equipment.

Flexography is an offset letterpress technique where the printing platesare made from rubber or photopolymers. The printing on pressuresensitive label is accomplished by the transfer of ink from the raisedsurface of the printing plate to the surface of the material beingprinted. The rotogravure method of printing uses a print cylinder withthousands of tiny cells which are below the surface of the printingcylinder. The ink is transferred from the cells when the print cylinderis brought into contact with the pressure sensitive label at theimpression roll. Printing inks for flexography or rotogravure includesolvent based inks, water based inks and radiation cured inks. Whilerotogravure and flexography printing do provide acceptable imagequality, these two printing methods require expensive and time consumingpreparation of print cylinders or printing plates which make printingjobs of less than 100,000 units expensive as the set up cost and thecost of the cylinders or printing plates is typically depreciated overthe size of the print job.

Recently, digital printing has become a viable method for the printingof information on packages. The term digital printing refers to theelectronic digital characters or electronic digital images that can beprinted by an electronic output device capable of translating digitalinformation. The two main digital printing technologies are ink jet andelectrophotography.

The introduction of piezo impulse drop-on-demand (DOD) and thermal DODinkjet printers in the early 1980's provided ink jet printing systems.These early printers were very slow, and the inkjet nozzles oftenclogged. In the 1990's Hewlett Packard introduced the first monochromeink jet printer, and, shortly thereafter, the introduction of color,wide format ink jet printers enabled businesses to enter the graphicarts market. Today, a number of different ink jet technologies are beingused for packaging, desktop, industrial, commercial, photographic, andtextile applications.

In piezo technology, a piezo crystal is electrically stimulated tocreate pressure waves, which eject ink from the ink chamber. The ink canbe electrically charged and deflected in a potential field, allowing thedifferent characters to be created. More recent developments haveintroduced DOD multiple jets that utilize conductive piezo ceramicmaterial, which, when charged, increases the pressure in the channel andforces a drop of ink from the end of the nozzle. This allows forvery-small droplets of ink to form and be delivered at high speed atvery high resolution, approximately 1,000 dpi printing.

Until recently, the use of color pigments in jet inks was uncommon.However, this is changing rapidly. Submicron pigments were developed inJapan for ink jet applications. Use of pigments allows for moretemperature resistant inks required for thermal ink jet printers andlaminations. Pigmented water-based jet inks are commercially available,and UV-curable jet inks are in development. Pigmented inks have greaterlightfastness and water-resistance.

Digital ink jet printing has the potential to revolutionize the printingindustry by making short-run, color print jobs more economical. However,the next commercial stage will require significant improvements in inkjet technology, the major hurdle remaining is to improve print speed.Part of this problem is the limitation of the amount of data the printercan handle rapidly. The more complex the design, the slower the printingprocess. Right now they are about 10 times slower than comparabledigital electrostatic printers.

Electrophotography was invented in the 1930's by Chester Carlson. By theearly 1970's, the development of an electrophotographic color copier wasbeing investigated by many companies. The technology for producing colorcopiers was already in place, but the market was not. It would take manymore years until customer demand for color copies would create thenecessary incentive to develop suitable electrostatic color copiers. Bythe late 1970's a few companies were using fax machines that could scana document, reduce the images to electronic signals, send them out overthe telephone wire, and, using another fax machine, retrieve theelectronic signals and print the original image using heat-sensitivepapers to produce a printed copy.

In 1993 Indigo and Xeikon introduced commercial digital printingmachines targeted on short-run markets that were dominated by sheet-fedlithographic printers. Elimination of intermediate steps associated withnegatives and plates used in offset printing provides faster turnaroundand better customer service. These digital presses share some of thecharacteristics of traditional xerography but use very specialized inks.Unlike inks for conventional photocopiers, these inks are made with verysmall particle size components in the range of 1 μm. Dry toners used inxerography are typically 8-10 μm in size.

In 1995 Indigo introduced the Ominus press designed for printingflexible packaging products. The Ominus uses a digital offset colorprocess called One Shot Color that has six colors. A key improvement hasbeen the use of a special white Electro ink for transparent substrates.The Ominus web-fed digital printing system allows printing of varioussubstrates using an offset cylinder that transfers the color image tothe substrate. In principle, this allows perfect register regardless ofthe substrate being printed; paper, film, and metal can be printed bythis process. This digital printing system is based on anelectrophotographic process where the electrostatic image is created onthe surface of a photoconductor by first charging the photo-conductor bycharge corona and exposing the photoconductive surface to a light sourcein image fashion.

The charged electrostatic latent image is then developed using inkcontaining an opposite charge to that on the image. This part of theprocess is similar to that of electrostatic toners associated withphoto-copying machines. The latent charged electrostatic image formed onthe photoconductor surface is developed by means of electrophoretictransfer of the liquid toner. This electrostatic toner image is thentransferred to a hot blanket, which coalesces the toner and maintains itin a tacky state until it is transferred to the substrate, which coolsthe ink and produces a tack-free print.

Electro inks typically comprise mineral oil and volatile organiccompounds below that of conventional offset printing inks. They aredesigned so that the thermoplastic resin will fuse at elevatedtemperatures. In the actual printing process, the resin coalesced, theinks are transferred to the substrate, and there is no need to heat theink to dry it. The ink is deposited on the substrate essentially dry,although it becomes tack-free as it cools and reaches room temperature.

For several decades a magnetic digital technology called “magnetography”has been under development. This process involves creating electricalimages on a magnetic cylinder and using magnetic toners as inks tocreate the image. The potential advantage of this technology lies in itshigh press speed. Tests have shown speeds of 200 meters per minute.Although these magnetic digital printers are limited to black and whitecopy, developments of color magnetic inks would make this high-speeddigital technology economically feasible. The key to its growth will befurther development of the VHSM (very high speed magnetic) drum and thecolor magnetic inks.

Within the magnetic digital arena, a hybrid system calledmagnetolithography has been built and tested on narrow web and short-runapplications developed by Nipson Printing Systems in Belfort, France.The technology appears to provide high resolution, and tests have beenconducted using a silicon-based, high density, magnetographic head. Muchmore work is necessary in the ink development to bring this system to acompetitive position relative to ink jet or electrophotography. However,the fact that it has high speed printing potential makes it anattractive alternate for packaging applications in which today's ink jetand electrophotography technologies are lagging.

Photographic materials have been known for use as prints for preservingmemories for special events such as birthdays and vacations. They alsohave been utilized for large display materials utilized in advertising.

Silver-halide photographic elements contain light sensitive silverhalide in a hydrophilic emulsion. An image is formed in the element byexposing the silver halide to light, or to other actinic radiation, anddeveloping the exposed silver halide to reduce it to elemental silver.

In color photographic elements, a dye image is formed as a consequenceof silver halide development by one of several different processes. Themost common is to allow a by-product of silver-halide development,oxidized silver-halide developing agent, to react with a compound calleda coupler to form the dye image. The silver and unreacted silver halideare then removed from the photographic element, leaving a dye image.

In either case, formation of the image commonly involves liquidprocessing with aqueous solutions that must penetrate the surface of theelement to come into contact with silver halide and coupler. Thus,gelatin and similar natural or synthetic hydrophilic polymers haveproven to be the binders of choice for silver-halide photographicelements.

A disadvantage of gelatin and other related hydrophilic colloids, isthat it is highly sensitive to relative humidity. While this is anadvantage during processing, a gelatin based coating such as in aphotographic element will have substantial residual tensile stress inthe dried coating and this residual stress causes curl toward theimaging side. The magnitude of the stress and the resultant curl is afunction of humidity and temperature of the environment. The curl ismost pronounced at low humidity environment when the equilibrium amountof moisture in the gelatin coating is low. As the humidity increases,the coating absorbs moisture from the atmosphere and the moistureplasticizes the coating and reduces the tensile stress in the coating.An anhydrous gelatin coating exhibits glass transition temperature (Tg)around 175° C. The Tg decreases as the humidity increases and it reachesroom temperature at 80% relative humidity. Assuming the substrate ismoisture insensitive, a pure gelatin coating will experience zero stressat 80% relative humidity(RH) and it will be under tensile stresswhenever the humidity falls below 80% RH. Such large changes in thermalcharacteristics and residual stresses at low relative humidity and hightemperatures can cause an adhesive based photographic label or print tocurl, and in extreme cases, lift off from the surface to which it ismounted, particularly from an untreated low surface energy media such ashigh density polyethylene (HDPE).

It is known in the art to coat the same hydrophilic binder on thebackside of conventional photographic elements to control curl inducedby hydrophilic colloids such as gelatin on the face side of the films.In all these cases the gelatin layer on the backside of the film iscomparable in thickness to the front side and is exposed to theenvironment as is the front side thereby enabling the atmosphere inducedcurl on the front to be balanced by the coating on the back. However, inthe case of a silver halide based print or label which has a pressuresensitive adhesive on the backside of the element furthest away from thebase, there still exists a need for providing robustness towards curlunder a variety of humidity and temperature conditions without goinginto the additional expense of providing laminates to achieve the same.

SUMMARY OF THE INVENTION

It is an object of the invention to overcome disadvantages of priorimage elements.

It is another object of the invention to form an imaging element withimproved curl properties.

The invention is generally accomplished by providing an image elementcomprising at least one image layer, a base, a gelatin layer below saidbase and a pressure sensitive adhesive below said gelatin layer, whereinsaid base has a stiffness of less than 20 mN.

DETAILED DESCRIPTION OF THE INVENTION

The invention has numerous advantages over prior practices in the art.The invention provides a printing method that is economically viablewhen printing short runs as the cost of printing plates or printingcylinders are avoided. The use of silver halide images for example,applied to a package ensures the highest image quality currentlyavailable compared to the common but lower quality six color rotogravureprinted images. Further, because the yellow, magenta, and cyan layerscontain gelatin interlayers, the silver halide images appear to havedepth. Silver halide image layers have also been optimized to accuratelyreplicate flesh tones, providing superior images of people compared toalternate prior art digital imaging technologies.

Silver halide image technology can simultaneously print text, graphics,and photographic quality images on the pressure sensitive label. Sincethe silver halide imaging layers of the invention are both optically anddigitally compatible, text, graphics, and images can be printed usingknown digital printing equipment such as lasers and CRT printers.Because the silver halide system is digitally compatible, each packagecan contain different data thereby enabling customization of individualpackages without the extra expense of printing plates or cylinders.Further, printing digital files allows the files to be transported usingelectronic data transfer technology such as the internet thus reducingthe cycle time to apply printing to a package. Silver halide imaginglayers can be digitally exposed with a laser or CRT at speeds greaterthan 75 meters per minute allowing competitive printing speeds comparedto current ink jet or electrophotographic printing engines. These andother advantages will be apparent from the detailed description below.

The present invention provides a novel way to control curl using abalanced architecture, at low humidities and high temperatures of thefinal label for flexible packaging material or a sticker printcomprising a hydrophilic imaged layer. In accordance with thisinvention, a gelatin layer is coated below the base on the side oppositethe silver halide light sensitive layer. This invention alsocontemplates a pressure sensitive adhesive layer over the curlcontrolling gelatin layer coated on the back side of the element awayfrom the base wherein said base has a stiffness of less than 20 mN.Stiffness less than 20 mN allows the base to be utilized for productlabeling of consumer goods and allows for the image to be adhered inphotographic albums as the 20 mN base is thin saving storage space inthe album.

In dry conditions, that is relative humidity less than 50%, the gelatinor other hydrophilic colloid layers utilized in photographic and inkjetimaging layers begin to contract, causing the base to be subjected to acurling force. Prior art imaging bases have solved this curling forceproblem by providing a stiff and thick base material for resisting thecurling force of the gelatin. By providing a gelatin layer opposite theimaging layer, the curing force is balanced without any visiblereduction in image quality or function.

An elastic modulus of the gelatin layer opposite the image is preferablygreater than 4000 MPa or more preferably between 4000 and 6500 MPa. Agelatin elastic modulus of the gelatin layer opposite the imaging layersgreater than 4000 MPa is preferred as is has been shown to balance thecurl of typical silver halide and ink jet imaging layers. An elasticmodulus of the gelatin less than 3500 MPa does not provide enough backcurl to offset the curling forces of a typical imaging element. Anelastic modulus of the gelatin layer opposite the imaging layer greaterthan 7000 begins to overwhelm the curling force of the gelatin utilizedin the imaging layers creating unacceptable curl toward the image. Thegelatin layer of the invention provides a means to balance the curlingforces caused by the contraction of the imaging layer gelatin to yield athin imaging element that is substantially flat. A flat imaging elementhas great commercial value in that 20 mN photographic pressure sensitivelabels will not fall off of packages particularly when the ambientrelative humidity is below 20% RH or curl at the time of labeldispensing. Further for consumer applications a 20 mN pressure sensitiveimaging element will not fall off of surfaces such as walls, windows andrefrigerators, surfaces that consumers typically adhere images.

The elastic modulus of the gelatin layers is measured by coating agelatin layer on a silicone or other release sheet. The dried andcrosslinked gelatin layer is later stripped from the release sheet andallowed to equilibrate to 20%RH at a temperature of 14° C. Then a 5 cmwide piece of the gelatin layer is measured on an Instron tenstiletester to determine the elastic modulus of the gelatin

The use of film-forming hydrophilic colloids as binders for silverhalide and other photographic addenda in imaging elements, includingphotographic films and photographic papers, is very well known. The mostcommonly used of these is gelatin and gelatin is a particularlypreferred material for use in this invention. It is used as the binderin the silver halide emulsion layer(s) and as the curl control layer.Useful gelatins include alkali-treated gelatin (cattle bone or hidegelatin), acid-treated gelatin (pigskin gelatin) and gelatin derivativessuch as acetylated gelatin, phthalated gelatin and the like. Otherhydrophilic colloids that can be utilized alone or in combination withgelatin include other proteins, protein derivatives, cellulosederivatives (e.g., cellulose esters), dextran, gum arabic, zein, casein,pectin, collagen derivatives, collodion, agar-agar, arrowroot, albumin,and the like. Still other useful hydrophilic colloids are water-solublepolyvinyl compounds such as polyvinyl alcohol, acrylamide polymers,poly(vinyl lactams), poly(vinylpyrrolidone), polyvinyl acetals, polymersof alkyl and sulfoalkyl acrylates and methacrylates, hydrolyzedpolyvinyl acetates, polyamides, polyvinyl pyridine, methacrylamidecopolymers, and the like.

The image element of the present invention is further incorporated witha gelatin or other hydrophilic colloid layer on the side of the baseopposite the imaging side to control the curl induced by the coating onthe front side. The thickness of the curl control layer can vary from0.1-10 microns, preferably from 0.5-5 microns to balance the curl of a3-25 micron hydrophilic coating on the front side.

In the practice of this invention the curl controlling layer on thebackside of the base is further coated with a pressure sensitiveadhesive (PSA) layer over it. The PSAs comprise acrylics, urethane andstyrenic polymers and copolymers, including natural rubbers. Thesehydrophobic polymers are coated from water or an organic solvent, in aformulation that contains, tackifiers, plasticizers and the like.

This invention further contemplates in another embodiment a systemwherein said pressure sensitive adhesive comprises between 20 and 40% byweight gelatin in the PSA layer.

The coating compositions of the invention can be applied by any of anumber of well known techniques, such as dip coating, rod coating, bladecoating, air knife coating, gravure coating and reverse roll coating,slot coating, extrusion coating, slide coating, curtain coating, and thelike after printing and processing the label and before application tocontainers utilizing high speed labeling equipment. After coating, thelayer is generally dried by simple evaporation, which may be acceleratedby known techniques such as convection heating. Known coating and dryingmethods are described in further detail in Research Disclosure No.308119, Published December 1989, pages 1007 to 1008.

A typical multicolor photographic element comprises a support bearing acyan dye image-forming unit comprised of at least one red-sensitivesilver halide emulsion layer having associated therewith at least onecyan dye-forming coupler, a magenta dye image-forming unit comprising atleast one green-sensitive silver halide emulsion layer having associatedtherewith at least one magenta dye-forming coupler, and a yellow dyeimage-forming unit comprising at least one blue-sensitive silver halideemulsion layer having associated therewith at least one yellowdye-forming coupler.

Suitable silver-halide emulsions and their preparation, as well asmethods of chemical and spectral sensitization, are described inSections I through V of Research Disclosures 37038 and 38957. Others aredescribed in U.S. Ser. No. 09/299,395, filed Apr. 26, 1999 and U.S. Ser.No. 09/299,548, filed Apr. 26, 1999, which are incorporated in theirentirety by reference herein. Color materials and development modifiersare described in Sections V through XX of Research Disclosures 37038 and38957. Vehicles are described in Section II of Research Disclosures37038 and 38957, and various additives such as brighteners,antifoggants, stabilizers, light absorbing and scattering materials,hardeners, coating aids, plasticizers, lubricants and matting agents aredescribed in Sections VI through X and XI through XIV of ResearchDisclosures 37038 and 38957. Processing methods and agents are describedin Sections XIX and XX of Research Disclosures 37038 and 38957, andmethods of exposure are described in Section XVI of Research Disclosures37038 and 38957.

In order to successfully transport materials of the invention, thereduction of static caused by web transport through manufacturing andimage processing is desirable. Since the light sensitive imaging layersof this invention can be fogged by light from a static dischargeaccumulated by the web as it moves over conveyance equipment such asrollers and drive nips, the reduction of static is necessary to avoidundesirable static fog. The polymer substrate materials of thisinvention have a marked tendency to accumulate static charge as theycontact machine components during transport. The use of an antistaticmaterial to reduce the accumulated charge on the web materials of thisinvention is desirable. Antistatic materials may be coated on the webmaterials of this invention and may contain any known materials in theart which can be coated on photographic web materials to reduce staticduring the transport of photographic paper. Examples of antistaticcoatings include conductive salts and colloidal silica. Desirableantistatic properties of the support materials of this invention mayalso be accomplished by antistatic additives which are an integral partof the polymer layer. Incorporation of additives that migrate to thesurface of the polymer to improve electrical conductivity include fattyquaternary ammonium compounds, fatty amines, and phosphate esters. Othertypes of antistatic additives are hygroscopic compounds such aspolyethylene glycols and hydrophobic slip additives that reduce thecoefficient of friction of the web materials. An antistatic coatingapplied to the opposite side from the image layer or incorporated intothe support's backside polymer layer is preferred. The backside ispreferred because the majority of the web contact during conveyance inmanufacturing and photoprocessing is on the backside. The backside isthe side not carrying the emulsion containing image forming layers. Thepreferred surface resistivity of the antistat coat at 50% RH is lessthan 10¹³ ohm/square. A surface resistivity of the antistat coat at 50%RH is less than 10¹³ ohm/square and has been shown to sufficientlyreduce static fog in manufacturing and during photoprocessing of theimage layers.

Conductive layers can be incorporated into multilayer imaging elementsin any of various configurations depending upon the requirements of thespecific imaging element. The conductive layer may be present as asubbing or tie layer underlying a magnetic recording layer on the sideof the support opposite the imaging layer(s). However, conductive layerscan be overcoated with layers other than a transparent magneticrecording layer (e.g., abrasion-resistant backing layer, curl controllayer, pelloid, etc.) in order to minimize the increase in theresistivity of the conductive layer after overcoating. Further,additional conductive layers also can be provided on the same side ofthe support as the imaging layer(s) or on both sides of the support. Anoptional conductive subbing layer can be applied either underlying oroverlying a gelatin subbing layer containing an antihalation dye orpigment. Alternatively, both antihalation and antistatic functions canbe combined in a single layer containing conductive particles,antihalation dye, and a binder. Such a hybrid layer is typically coatedon the same side of the support as the sensitized emulsion layer.Additional optional layers can be present as well. An additionalconductive layer can be used as an outermost layer of an imagingelement, for example, as a protective layer overlying an image-forminglayer. When a conductive layer is applied over a sensitized emulsionlayer, it is not necessary to apply any intermediate layers such asbarrier or adhesion-promoting layers between the conductive overcoatlayer and the imaging layer(s), although they can optionally be present.Other addenda, such as polymer latices to improve dimensional stability,hardeners or cross-linking agents, surfactants, matting agents,lubricants, and various other well-known additives can be present in anyor all of the above mentioned layers.

Conductive layers underlying a transparent magnetic recording layertypically exhibit an internal resistivity of less than 1×10¹⁰ohms/square, preferably less than 1×10⁹ ohms/square, and morepreferably, less than 1×10⁸ ohms/square.

The terms as used herein, “top”, “upper”, “emulsion side”, and “face”mean the side or toward the side of a packaging material bearing theimaging layers. The term environmental protection layer means the layerapplied over the imaging layers after image formation. The terms “facestock”, “substrate” and “base” mean the material to which thehydrophilic imaging layers such as silver halide layers are applied. Theterms “bottom”, “lower side”, and “back” mean the side or toward theside of the label or packaging material opposite from the side bearingthe images formed in a gelatin media.

In order to produce a pressure sensitive photographic label, the linermaterial that carries the pressure sensitive adhesive, face stock andimaged layers, the liner material must allow for efficient transport inmanufacturing, image printing, image development, label converting andlabel application equipment. A label comprising a silver halide imaginglayer, a base and a strippable liner connected by an adhesive to saidbase, wherein said base has a stiffness of between 15 and 60 mN and anL* is greater than 92.0, and wherein said liner has a stiffness ofbetween 40 and 120 mN is preferred. The photographic label packagingmaterial is preferred with the white, stiff liner as it allows forefficient transport through photographic printing and processingequipment and improves printing speed compared to typical linermaterials that are brown or clear and have little contribution tosecondary exposure.

A peelable liner or back is preferred as the pressure sensitive adhesiverequired for adhesion of the label to the package, can not betransported through labeling equipment without the liner. The linerprovides strength for conveyance and protects the pressure sensitiveadhesive prior to application to the package. A preferred liner materialis cellulose paper. A cellulose paper liner is flexible, strong and lowin cost compared to polymer substrates. Further, a cellulose papersubstrate allows for a textured label surface that can be desirable insome packaging applications. The paper may be provided with coatingsthat will provide waterproofing to the paper as the photographic elementof the invention must be processed in aqueous chemistry to develop theimage. Examples of a suitable waterproof coatings applied to the paperare acrylic polymer, melt extruded polyethylene and oriented polyolefinsheets laminated to the paper. Paper is also preferred as paper cancontain moisture and salt which provide antistatic properties thatprevent static sensitization of the silver halide image layers.

Further, paper containing sizing agents, known in the photographic paperart and disclosed in U.S. Pat. No. 6,093,521, provide resistance to edgepenetration of the silver halide image processing chemistry. An edgepenetration of less than 8 micrometers is preferred as processingchemistry penetrated into the paper greater than 12 micrometers has beenshown to swell causing die cutting problems when face stock matrix isdie cut and stripped from the liner. Also, penetration of processingchemistry greater than 12 micrometers increases the chemistry usage inprocessing resulting in higher processing costs.

Another preferred liner material or peelable back is an oriented sheetof polymer. The liner preferably is an oriented polymer because of thestrength and toughness developed in the orientation process. Preferredpolymers for the liner substrate include polyolefins, polyester andnylon. Preferred polyolefin polymers include polypropylene,polyethylene, polymethylpentene, polystyrene, polybutylene, and mixturesthereof. Polyolefin copolymers, including copolymers of propylene andethylene such as hexene, butene, and octene are also useful. Polyesteris most preferred, as it is has desirable strength and toughnessproperties required for efficient transport of silver halide pressuresensitive label liner in high speed labeling equipment.

In another preferred embodiment, the liner consists of a paper core towhich sheets of oriented polymer are laminated. The laminated paperliner is preferred because the oriented sheets of polymer providetensile strength which allows the thickness of the liner to be reducedcompared to coated paper and the oriented polymer sheet providesresistance to curl during manufacturing and drying in the silver halideprocess.

The tensile strength of the liner or the tensile stress at which asubstrate breaks apart is an important conveyance and forming parameter.Tensile strength is measured by ASTM D882 procedure. A tensile strengthgreater than 120 MPa is preferred as liners less than 110 MPa begin tofracture in automated packaging equipment during conveyance, forming andapplication to the package.

The coefficient of friction or COF of the liner bearing the silverhalide imaging layer is an important characteristic as the COF isrelated to conveyance and forming efficiency in automated labelingequipment. COF is the ratio of the weight of an item moving on a surfaceto the force that maintains contact between the surface and the item.The mathematical expression for COF is as follows:

COF=μ=(friction force/normal force)

The COF of the liner is measured using ASTM D-1894 utilizing a stainlesssteel sled to measure both the static and dynamic COF of the liner. Thepreferred COF for the liner of the invention is between 0.2 and 0.6. Asan example, a 0.2 COF is necessary for coating on a label used in apick-and-place application. The operation using a mechanical device topick a label and move it to another point requires a low COF so thelabel will easily slide over the surface of the label below it. At theother extreme, large sheets such as book covers require a 0.6 COF toprevent them from slipping and sliding when they are piled on top ofeach other in storage. Occasionally, a particular material may require ahigh COF on one side and a low COF on the other side. Normally, the basematerial itself, such as a plastic film, foil, or paper substrate, wouldprovide the necessary COF for one side. Application of an appropriatecoating would modify the image side to give the higher or lower value.Conceivably, two different coatings could be used with one on eitherside. COF can be static or kinetic. The coefficient of static frictionis the value at the time movement between the two surfaces is ready tostart but no actual movement has occurred. The coefficient of kineticfriction refers to the case when the two surfaces are actually slidingagainst each other at a constant rate of speed. COF is usually measuredby using a sled placed on the surface. The force necessary at the onsetof sliding provides a measurement of static COF. Pulling the sled at aconstant speed over a given length provides a measure of kineticfrictional force.

The preferred thickness of the liner of the invention is between 75 and225 micrometers. Thickness of the liner is important in that thestrength of the liner, expressed in terms of tensile strength ormechanical modulus, must be balanced with the thickness of the liner toachieve a cost efficient design. For example, thick liners that are highin strength are not cost efficient because thick liners will result inshort roll lengths compared to thin liners at a given roll diameter. Aliner thickness less that 60 micrometer has been shown to causetransport failure in the edge guided silver halide printers. A linerthickness greater than 250 micrometers yields a design that is not costeffective and is difficult to transport in existing silver halideprinters.

The liner of the invention preferably has an optical transmission ofless than 20%. During the printing of the silver halide labels, exposurelight energy is required to reflect from the face stock/linercombination to yield a secondary exposure. This secondary exposure iscritical to maintaining high level of printing productivity. It has beenshown that liners with an optical transmission of greater than 25%significantly reduces the printing speed of the silver halide label.Further, clear face stock material to provide the “no label look” needan opaque liner to not only maintain printing speed, but to preventunwanted reflection from printing platens in current silver halideprinters.

Since the light sensitive silver halide layers with expanded color gamutcan suffer from unwanted exposure from static discharge duringmanufacturing, printing and processing, the liner preferably has aresistivity of less than 10¹¹ ohms/square. A wide variety ofelectrically-conductive materials can be incorporated into antistaticlayers to produce a wide range of conductivities. These can be dividedinto two broad groups: (i) ionic conductors and (ii) electronicconductors. In ionic conductors charge is transferred by the bulkdiffusion of charged species through an electrolyte. Here theresistivity of the antistatic layer is dependent on temperature andhumidity. Antistatic layers containing simple inorganic salts, alkalimetal salts of surfactants, ionic conductive polymers, polymericelectrolytes containing alkali metal salts, and colloidal metal oxidesols (stabilized by metal salts), described previously in patentliterature, fall in this category. However, many of the inorganic salts,polymeric electrolytes, and low molecular weight surfactants used arewater-soluble and are leached out of the antistatic layers duringprocessing, resulting in a loss of antistatic function. The conductivityof antistatic layers employing an electronic conductor depends onelectronic mobility rather than ionic mobility and is independent ofhumidity. Antistatic layers which contain conjugated polymers,semiconductive metal halide salts, semiconductive metal oxide particles,etc. have been described previously. However, these antistatic layerstypically contain a high volume percentage of electronically conductingmaterials which are often expensive and impart unfavorable physicalcharacteristics, such as color, increased brittleness, and poor adhesionto the antistatic layer.

In a preferred embodiment of this invention the label has an antistatmaterial incorporated into the liner or coated on the liner. It isdesirable to have an antistat that has an electrical surface resistivityof at least 10¹¹ log ohms/square. In the most preferred embodiment, theantistat material comprises at least one material selected from thegroup consisting of tin oxide and vanadium pentoxide.

In another preferred embodiment of the invention antistatic material areincorporated into the pressure sensitive adhesive layers. The antistaticmaterial incorporated into the pressure sensitive adhesive layerprovides static protection to the silver halide layers and reduces thestatic on the photographic label which has been shown to aid labeling ofcontainers in high speed labeling equipment. As a stand-alone orsupplement to the liner comprising an antistatic layer, the pressuresensitive adhesive may also further comprise an antistatic agentselected from the group consisting of conductive metal oxides, carbonparticles, and synthetic smectite clay, or multi-layered with aninherently conductive polymer. In one of the preferred embodiments, theantistat material is metal oxides. Metal oxides are preferred becausethey are readily dispersed in the thermoplastic adhesive and can beapplied to the polymer sheet by any means known in the art. Conductivemetal oxides that may be useful in this invention are selected from thegroup consisting of conductive particles including doped-metal oxides,metal oxides containing oxygen deficiencies, metal antimonates,conductive nitrides, carbides, or borides, for example, TiO₂, SnO₂,Al₂O₃, ZrO₃, In₂O₃, MgO, ZnSb₂O₆, InSbO₄, TiB₂, ZrB₂, NbB₂, TaB₂, CrB₂,MoB, WB, LaB₆, ZrN, TiN, TiC, and WC. The most preferred materials aretin oxide and vanadium pentoxide because they provide excellentconductivity and are transparent.

The base material, or the flexible substrate utilized in this inventionon to which the light sensitive silver halide imaging layers areapplied, must not interfere with the silver halide imaging layers.Further, the base material of this invention needs to optimize theperformance of the silver halide imaging system. Suitable flexiblesubstrates must also perform efficiently in a automated packagingequipment for the application of photographic labels to variouscontainers. A preferred flexible substrate is cellulose paper. Acellulose paper substrate is flexible, strong and low in cost comparedto polymer substrates. Further, a cellulose paper substrate allows for atextured photographic label surface that can be desirable in somepackaging applications. The paper may preferably be provided withcoatings that will provide waterproofing to the paper as thephotographic element of the invention must be processed in aqueouschemistry to develop the silver halide image. An example of a suitablecoating is acrylic or polyethylene polymer.

Polymer substrates are another preferred base material because they aretear resistant, have excellent conformability, good chemical resistanceand are high in strength. Preferred polymer substrates includepolyester, oriented polyolefin such as polyethylene and polypropylene,cast polyolefins such as polypropylene and polyethylene, polystyrene,acetate and vinyl. Polymers are preferred as they are strong andflexible and provide an excellent surface for the coating of silverhalide imaging layers.

Biaxially oriented polyolefin sheets are preferred as they are low incost, have excellent optical properties that optimize the silver halidesystem and can be applied to packages in high speed labeling equipment.Microvoided composite biaxially oriented sheets are most preferredbecause the voided layer provides opacity and lightness without the needfor TiO₂. Also, the voided layers of the microvoided biaxially orientedsheets have been shown to significantly reduce pressure sensitivity ofthe silver halide imaging layers. Microvoided biaxially oriented sheetsare conveniently manufactured by coextrusion of the core and surfacelayers, followed by biaxial orientation, whereby voids are formed aroundvoid-initiating material contained in the core layer. Such compositesheets are disclosed in U.S. Pat. Nos. 4,377,616; 4,758,462, 4,632,869and 5,866,282. The biaxially oriented polyolefin sheets also may belaminated to one or both sides of a paper sheet to form a photographiclabel with greater stiffness if that is needed.

The flexible polymer base substrate may contain more than one layer. Theskin layers of the flexible substrate can be made of the same polymericmaterials as listed above for the core matrix. The composite sheet canbe made with skin(s) of the same polymeric material as the core matrix,or it can be made with skin(s) of different polymeric composition thanthe core matrix. For compatibility, an auxiliary layer can be used topromote adhesion of the skin layer to the core.

Voided biaxially oriented polyolefin sheets are a preferred flexiblebase substrate for the coating of light sensitive silver halide imaginglayers. Voided films are preferred as they provide opacity, whitenessand image sharpness to the image. “Void” is used herein to mean devoidof added solid and liquid matter, although it is likely the “voids”contain gas. The void-initiating particles which remain in the finishedpackaging sheet core should be from 0.1 to 10 μm in diameter andpreferably round in shape to produce voids of the desired shape andsize. The size of the void is also dependent on the degree oforientation in the machine and transverse directions. Ideally, the voidwould assume a shape which is defined by two opposed and edge contactingconcave disks. In other words, the voids tend to have a lens-like orbiconvex shape. The voids are oriented so that the two major dimensionsare aligned with the machine and transverse directions of the sheet. TheZ-direction axis is a minor dimension and is roughly the size of thecross diameter of the voiding particle. The voids generally tend to beclosed cells, and thus there is virtually no path open from one side ofthe voided-core to the other side through which gas or liquid cantraverse.

The photographic element of this invention generally has a glossysurface, that is, a surface that is sufficiently smooth to provideexcellent reflection properties.

A nacreous reflective base is a preferred embodiment because it providesa unique photographic appearance to a photographic label that isperceptually preferred by consumers. The opalescent, nacreous appearanceis achieved when the microvoids in the vertical direction of the basesheet are between 1 and 3 μm. By the vertical direction, it is meant thedirection that is perpendicular to the plane of the imaging member. Thethickness of the microvoids preferably is between 0.7 and 1.5 μm forbest physical performance and opalescent properties. The preferrednumber of microvoids in the vertical direction is between 8 and 30. Lessthan 6 microvoids in the vertical direction do not create the desiredopalescent surface. Greater than 35 microvoids in the vertical directiondo not significantly improve the optical appearance of the opalescentsurface.

The void-initiating material for the flexible base substrate may beselected from a variety of materials and should be present in an amountof about 5 to 50% by weight based on the weight of the core matrixpolymer. Preferably, the void-initiating material comprises a polymericmaterial. When a polymeric material is used, it may be a polymer thatcan be melt-mixed with the polymer from which the core matrix is madeand be able to form dispersed spherical particles as the suspension iscooled down. Examples of this would include nylon dispersed inpolypropylene, polybutylene terephthalate in polypropylene, orpolypropylene dispersed in polyethylene terephthalate. If the polymer ispreshaped and blended into the matrix polymer, the importantcharacteristic is the size and shape of the particles. Spheres arepreferred and they can be hollow or solid. These spheres may be madefrom cross-linked polymers which are members selected from the groupconsisting of an alkenyl aromatic compound having the general formulaAr—C(R)═CH₂, wherein Ar represents an aromatic hydrocarbon radical, oran aromatic halohydrocarbon radical of the benzene series and R ishydrogen or the methyl radical acrylate-type monomers include monomersof the formula CH₂═C(R′)—C(O)(OR) wherein R is selected from the groupconsisting of hydrogen and an alkyl radical containing from about 1 to12 carbon atoms and R′ is selected from the group consisting of hydrogenand methyl; copolymers of vinyl chloride and vinylidene chloride,acrylonitrile and vinyl chloride, vinyl bromide, vinyl esters havingformula CH₂═CH(O)COR, wherein R is an alkyl radical containing from 2 to18 carbon atoms, acrylic acid, methacrylic acid, itaconic acid,citraconic acid, maleic acid, fumaric acid, oleic acid, vinylbenzoicacid; the synthetic polyester resins which are prepared by reactingterephthalic acid and dialkyl terephthalics or ester-forming derivativesthereof, with a glycol of the series HO(CH₂)_(n)OH wherein n is a wholenumber within the range of 2-10 and having reactive olefinic linkageswithin the polymer molecule, the above-described polyesters whichinclude copolymerized therein up to 20 percent by weight of a secondacid or ester thereof having reactive olefinic unsaturation and mixturesthereof, and a cross-linking agent selected from the group consisting ofdivinylbenzene, diethylene glycol dimethacrylate, diallyl fumarate,diallyl phthalate, and mixtures thereof.

Examples of typical monomers for making the cross-linked polymer voidinitiating particles include styrene, butyl acrylate, acrylamide,acrylonitrile, methyl methacrylate, ethylene glycol dimethacrylate,vinyl pyridine, vinyl acetate, methyl acrylate, vinylbenzyl chloride,vinylidene chloride, acrylic acid, divinylbenzene,acrylamidomethyl-propane sulfonic acid, vinyl toluene, etc. Preferably,the cross-linked polymer is polystyrene or poly(methyl methacrylate).Most preferably, it is polystyrene, and the cross-linking agent isdivinylbenzene.

Processes well known in the art yield nonuniformly sized void initiatingparticles, characterized by broad particle size distributions. Theresulting beads can be classified by screening the beads spanning therange of the original distribution of sizes. Other processes such assuspension polymerization, limited coalescence, directly yield veryuniformly sized particles.

The void-initiating materials may be coated with agents to facilitatevoiding. Suitable agents or lubricants include colloidal silica,colloidal alumina, and metal oxides such as tin oxide and aluminumoxide. The preferred agents are colloidal silica and alumina, mostpreferably, silica. The cross-linked polymer having a coating of anagent may be prepared by procedures well known in the art. For example,conventional suspension polymerization processes wherein the agent isadded to the suspension is preferred. As the agent, colloidal silica ispreferred.

The void-initiating particles can also be inorganic spheres, includingsolid or hollow glass spheres, metal or ceramic beads or inorganicparticles such as clay, talc, barium sulfate, or calcium carbonate. Theimportant thing is that the material does not chemically react with thecore matrix polymer to cause one or more of the following problems: (a)alteration of the crystallization kinetics of the matrix polymer, makingit difficult to orient, (b) destruction of the core matrix polymer, (c)destruction of the void-initiating particles, (d) adhesion of thevoid-initiating particles to the matrix polymer, or (e) generation ofundesirable reaction products, such as toxic or high color moieties. Thevoid-initiating material should not be photographically active ordegrade the performance of the photographic element in which thebiaxially oriented polyolefin sheet is utilized.

The total thickness of the topmost skin layer of the polymeric basesubstrate may be between 0.20 μm and 1.5 μm, preferably between 0.5 and1.0 μm. Below 0.5 μm any inherent nonplanarity in the coextruded skinlayer may result in unacceptable color variation. At skin thicknessgreater than 1.0 μm, there is a reduction in the photographic opticalproperties such as image resolution. At thickness greater than 1.0 μm,there is also a greater material volume to filter for contamination suchas clumps or poor color pigment dispersion.

Addenda may be added to the top most skin layer of the flexible basesubstrate to change the color of the imaging element. For labeling use,a white substrate with a slight bluish tinge is preferred. The additionof the slight bluish tinge may be accomplished by any process which isknown in the art including the machine blending of color concentrateprior to extrusion and the melt extrusion of blue colorants that havebeen preblended at the desired blend ratio. Colored pigments that canresist extrusion temperatures greater than 320° C. are preferred, astemperatures greater than 320° C. are necessary for coextrusion of theskin layer. Blue colorants used in this invention may be any colorantthat does not have an adverse impact on the imaging element. Preferredblue colorants include Phthalocyanine blue pigments, Cromophtal bluepigments, Irgazin blue pigments, and Irgalite organic blue pigments.Optical brightener may also be added to the skin layer to absorb UVenergy and emit light largely in the blue region. TiO₂ may also be addedto the skin layer. While the addition of TiO₂ in the thin skin layer ofthis invention does not significantly contribute to the opticalperformance of the sheet, it can cause numerous manufacturing problemssuch as extrusion die lines and spots. The skin layer substantially freeof TiO₂ is preferred. TiO₂ added to a layer between 0.20 and 1.5 μm doesnot substantially improve the optical properties of the support, willadd cost to the design, and will cause objectionable pigments lines inthe extrusion process.

Addenda may be added to the core matrix and/or to one or more skinlayers to improve the optical properties of the flexible substrate.Titanium dioxide is preferred and is used in this invention to improveimage sharpness or MTF, opacity, and whiteness. The TiO₂ used may beeither anatase or rutile type. Further, both anatase and rutile TiO₂ maybe blended to improve both whiteness and sharpness. Examples of TiO₂that are acceptable for a photographic system are DuPont Chemical Co.R101 rutile TiO₂ and DuPont Chemical Co. R104 rutile TiO₂. Otherpigments known in the art to improve photographic optical responses mayalso be used in this invention. Examples of other pigments known in theart to improve whiteness are talc, kaolin, CaCO₃, BaSO₄, ZnO, TiO₂, ZnS,and MgCO₃. The preferred TiO₂ type is anatase, as anatase TiO₂ has beenfound to optimize image whiteness and sharpness with a voided layer.

The voids provide added opacity to the flexible substrate. This voidedlayer can also be used in conjunction with a layer that contains atleast one pigment from the group consisting of TiO₂, CaCO₃, clay, BaSO₄,ZnS, MgCO₃, talc, kaolin, or other materials that provide a highlyreflective white layer in said film of more than one layer. Thecombination of a pigmented layer with a voided layer provides advantagesin the optical performance of the final image.

The flexible biaxially base substrate of this invention which has amicrovoided core is preferred. The microvoided core adds opacity andwhiteness to the imaging support, further improving imaging quality.Combining the image quality advantages of a microvoided core with amaterial, which absorbs ultraviolet energy and emits light in thevisible spectrum, allows for the unique optimization of image quality,as the image support can have a tint when exposed to ultraviolet energyyet retain excellent whiteness when the image is viewed using lightingthat does not contain significant amounts of ultraviolet energy such asindoor lighting.

It has been found that the microvoids located in the voided layer of theflexible biaxially oriented substrate provide a reduction in undesirablepressure fog. Mechanical pressure, of the order of hundreds of kilogramsper square centimeter, causes an undesirable, reversible decrease insensitivity by a mechanism at the time of writing that is not fullyunderstood. The net result of mechanical pressure is an unwantedincrease in density, mainly yellow density. The voided layer in thebiaxially oriented flexible substrate absorbs mechanical pressure bycompression of the voided layer, common in the converting andphotographic processing steps, and reduces the amount of yellow densitychange. Pressure sensitivity is measured by applying a 206 MPa load tothe coated light sensitive silver halide emulsion, developing the yellowlayer, and measuring the density difference with an X-Rite model 310 (orcomparable) photographic transmission densitometer between the controlsample which was unloaded and the loaded sample. The preferred change inyellow layer density is less than 0.02 at a pressure of 206 MPa. A 0.04change in yellow density is perceptually significant and, thus,undesirable.

The coextrusion, quenching, orienting, and heat setting of the flexiblebase substrate may be effected by any process which is known in the artfor producing oriented sheet, such as by a flat sheet process or abubble or tubular process. The flat sheet process involves extruding theblend through a slit die and rapidly quenching the extruded web upon achilled casting drum so that the core matrix polymer component of thesheet and the skin components(s) are quenched below their glasssolidification temperature. The quenched sheet is then biaxiallyoriented by stretching in mutually perpendicular directions at atemperature above the glass transition temperature and below the meltingtemperature of the matrix polymers. The sheet may be stretched in onedirection and then in a second direction or may be simultaneouslystretched in both directions. After the sheet has been stretched, it isheat set by heating to a temperature sufficient to crystallize or annealthe polymers, while restraining to some degree the sheet againstretraction in both directions of stretching.

By having at least one nonvoided skin on the microvoided core, thetensile strength of the flexible base substrate is increased and makesthe sheet more manufacturable. The higher tensile strength also allowsthe sheets to be made at wider widths and higher draw ratios than whensheets are made with all layers voided. Coextruding the layers furthersimplifies the manufacturing process.

In order to provide a digital printing technology that can be applied toa package that is high in quality, can handle text, graphic and images,is economical for short run printing jobs and accurately reproduce fleshtones, silver halide imaging is preferred. The silver halide technologycan be either black and white or color. The silver halide imaging layersare preferably exposed and developed prior to application to a package.The flexible substrate of the invention contains the necessary tensilestrength properties and coefficient of friction properties to allow forefficient transport and application of the images in high speed labelingequipment. The substrate of the invention is formed by applying lightsensitive silver halide imaging layers of a flexible label stock thatcontains a hydrophilic colloid based curl control layer and a pressuresensitive adhesive. The imaging layers, face stock and pressuresensitive adhesive are supported and transported through labelingequipment using a tough liner material.

In another embodiment of the invention, the base material is nacreous inappearance. For the imaging element of the invention, imaging layers areapplied to the top-side of the nacreous base. The nacreous basecomprises voided polymer layer below the imaging layers. The layersabove the voided layer and below the imaging layers are substantiallyfree of white pigments that have been shown to corrupt the dye hue inks,pigments or dyes used to form an image. Polymer layers below the voidedlayer do contain white, reflecting pigments, which have been shown tosignificantly improve sharpness, whiteness and photographic printingspeed compared to prior art materials.

While silver halide images are preferred for the above mentionedreasons, the environmental protection layer of the invention may also beutilized with other imaging materials such as inkjet, thermal,electrophotographic and the like. It particularly finds use with thosematerials that have a water soluble colloidal binder such as gelatin,polyvinyl alcohol etc.

Ink jet printing is a non-impact method for producing images by thedeposition of ink droplets in a pixel-by-pixel manner to animage-recording element in response to digital signals. There arevarious methods which may be utilized to control the deposition of inkdroplets on the image-recording element to yield the desired image. Inone process, known as continuous ink jet, a continuous stream ofdroplets is charged and deflected in an imagewise manner onto thesurface of the image-recording element, while unimaged droplets arecaught and returned to an ink sump. In another process, known asdrop-on-demand ink jet, individual ink droplets are projected as neededonto the image-recording element to form the desired image. Commonmethods of controlling the projection of ink droplets in drop-on-demandprinting include piezoelectric transducers and thermal bubble formation.Ink jet printers have found broad applications across markets rangingfrom industrial labeling to short run printing to desktop document andpictorial imaging.

The inks used in the various ink jet printers can be classified aseither dye-based or pigment-based. A dye is a colorant which ismolecularly dispersed or solvated by a carrier medium. The carriermedium can be a liquid or a solid at room temperature. A commonly usedcarrier medium is water or a mixture of water and organic co-solvents.Each individual dye molecule is surrounded by molecules of the carriermedium. In dye-based inks, no particles are observable under themicroscope. Although there have been many recent advances in the art ofdye-based ink jet inks, such inks still suffer from deficiencies such aslow optical densities on plain paper and poor light-fastness. When wateris used as the carrier medium, such inks also generally suffer from poorwater-fastness.

An ink jet recording element typically comprises a support having on atleast one surface thereof an ink-receiving or image-forming layer. Theink-receiving layer may be a polymer layer which swells to absorb theink or a porous layer which imbibes the ink via capillary action.

Ink jet prints, prepared by printing onto ink jet recording elements,are subject to environmental degradation. They are especially vulnerableto water smearing, dye bleeding, coalescence and light fade. Forexample, since ink jet dyes are water-soluble, they can migrate fromtheir location in the image layer when water comes in contact with thereceiver after imaging. Highly swellable hydrophilic layers can take anundesirably long time to dry, slowing printing speed, and will dissolvewhen left in contact with water, destroying printed images. Porouslayers speed the absorption of the ink vehicle, but often suffer frominsufficient gloss and severe light fade.

A binder may also be employed in the image-receiving layer in theinvention. In a preferred embodiment, the binder is a water solublecolloidal polymer. Examples of water soluble colloidal polymers usefulin the invention include poly(vinyl alcohol), polyvinylpyrrolidone,poly(ethyl oxazoline), poly-N-vinylacetamide, non-deionized or deionizedType IV bone gelatin, acid processed ossein gelatin, pig skin gelatin,acetylated gelatin, phthalated gelatin, oxidized gelatin, chitosan,poly(alkylene oxide), sulfonated polyester, partially hydrolyzedpoly(vinyl acetate-co-vinyl alcohol), poly(acrylic acid),poly(1-vinylpyrrolidone), poly(sodium styrene sulfonate),poly(2-acrylamido-2-methane sulfonic acid), polyacrylamide or mixturesthereof. In a preferred embodiment of the invention, the binder isgelatin or polyvinyl alcohol.

If a hydrophilic polymer is used in the image-receiving layer, it may bepresent in an amount of from about 0.02 to about 30 g/m², preferablyfrom about 0.04 to about 16 g/m² of the image-receiving layer.

Latex polymer particles and/or inorganic oxide particles may also beused as the binder in the image-receiving layer to increase the porosityof the layer and improve the dry time. Preferably the latex polymerparticles and/or inorganic oxide particles are cationic or neutral.Examples of inorganic oxide particles include barium sulfate, calciumcarbonate, clay, silica or alumina, or mixtures thereof. In that case,the weight percent of particulates in the image receiving layer is fromabout 80 to about 95%, preferably from about 85 to about 90%.

The pH of the aqueous ink compositions employed in the invention may beadjusted by the addition of organic or inorganic acids or bases. Usefulinks may have a preferred pH of from about 2 to 10, depending upon thetype of dye being used. Typical inorganic acids include hydrochloric,phosphoric and sulfuric acids. Typical organic acids includemethanesulfonic, acetic and lactic acids. Typical inorganic basesinclude alkali metal hydroxides and carbonates. Typical organic basesinclude-ammonia, triethanolamine and tetramethylethlenediamine.

A humectant is employed in the inkjet composition employed in theinvention to help prevent the ink from drying out or crusting in theorifices of the printhead. Examples of humectants which can be usedinclude polyhydric alcohols, such as ethylene glycol, diethylene glycol,triethylene glycol, propylene glycol, tetraethylene glycol, polyethyleneglycol, glycerol, 2-methyl-2,4-pentanediol 1,2,6-hexanetriol andthioglycol; lower alkyl mono- or di-ethers derived from alkyleneglycols, such as ethylene glycol mono-methyl or mono-ethyl ether,diethylene glycol mono-methyl or mono-ethyl ether, propylene glycolmono-methyl or mono-ethyl ether, triethylene glycol mono-methyl ormono-ethyl ether, diethylene glycol di-methyl or di-ethyl ether, anddiethylene glycol monobutylether, nitrogen-containing cyclic compounds,such as pyrrolidone, N-methyl-2-pyrrolidone, and1,3-dimethyl-2-imidazolidinone, and sulfur-containing compounds such asdimethyl sulfoxide and tetramethylene sulfone. A preferred humectant forthe composition employed in the invention is diethylene glycol,glycerol, or diethylene glycol monobutylether.

Water-miscible organic solvents may also be added to the aqueous inkemployed in the invention to help the ink penetrate the receivingsubstrate, especially when the substrate is a highly sized paper.Examples of such solvents include alcohols, such as methyl alcohol,ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol,sec-butyl alcohol, t-butyl alcohol, iso-butyl alcohol, furfuryl alcohol,and tetrahydrofurfuryl alcohol; ketones or ketoalcohols such as acetone,methyl ethyl ketone and diacetone alcohol; ethers, such astetrahydrofuran and dioxane; and esters, such as, ethyl lactate,ethylene carbonate and propylene carbonate.

Surfactants may be added to adjust the surface tension of the ink to anappropriate level. The surfactants may be anionic, cationic, amphotericor nonionic.

A biocide may be added to the composition employed in the invention tosuppress the growth of microorganisms such as molds, fungi, etc. inaqueous inks. A preferred biocide for the ink composition employed inthe present invention is Proxel® GXL (Zeneca Specialties Co.) at a finalconcentration of 0.0001-0.5 wt. %.

A typical ink composition employed with the imaging element of theinvention may comprise, for example, the following substituents byweight: colorant (0.05-5%), water (20-95%), a humectant (5-70%), watermiscible co-solvents (2-20%), surfactant,(0.1-10%), biocide (0.05-5%)and pH control agents (0.1-10%).

Additional additives which may optionally be present in the ink jet inkcomposition employed in the invention include thickeners, conductivityenhancing agents, anti-kogation agents, drying agents, and defoamers.

The ink jet inks employed utilizing the imaging element of thisinvention may be employed in ink jet printing wherein liquid ink dropsare applied in a controlled fashion to an ink receptive layer substrate,by ejecting ink droplets from a plurality of nozzles or orifices of theprint head of an inkjet printer.

The image-recording layer used in the imaging element of the presentinvention can also contain various known additives, including mattingagents such as titanium dioxide, zinc oxide, silica and polymeric beadssuch as crosslinked poly(methyl methacrylate) or polystyrene beads forthe purposes of contributing to the non-blocking characteristics and tocontrol the smudge resistance thereof; surfactants such as non-ionic,hydrocarbon or fluorocarbon surfactants or cationic surfactants, such asquaternary ammonium salts; fluorescent dyes; pH controllers;anti-foaming agents; lubricants; preservatives; viscosity modifiers;dye-fixing agents; waterproofing agents; dispersing agents; UV-absorbingagents; mildew-proofing agents; mordants; antistatic agents,anti-oxidants, optical brighteners, and the like. A hardener may also beadded to the ink-receiving layer if desired.

In order to improve the adhesion of the image-recording layer to thesupport, the surface of the support may be subjected to acorona-discharge-treatment prior to applying the image-recording layer.

In addition, a subbing layer, such as a layer formed from a halogenatedphenol or a partially hydrolyzed vinyl chloride-vinyl acetate copolymercan be applied to the surface of the support to increase adhesion of theimage recording layer. If a subbing layer is used, it should have athickness (i.e., a dry coat thickness) of less than about 2 μm.

The ink jet image-recording layer may be present in any amount which iseffective for the intended purpose. In general, good results areobtained when it is present in an amount of from about 2 to about 44g/m², preferably from about 6 to about 32 g/m², which corresponds to adry thickness of about 2 to about 40 μm, preferably about 6 to about 30μm for good balance of ink absorption, dry time and material usage.

The following examples are used to illustrate the present invention.However, it should be understood that the invention is not limited tothese illustrative examples.

EXAMPLES

All the coatings were made on a label face stock using the formulationand architecture described below.

Examples 1-4

Protoype for a silver halide pressure sensitive packaging labels werecreated by applying a 7.5 micrometer thick gelatin(Type IV, deioinzed)layer to the face side of a label stock which consisted of a flexiblewhite biaxially oriented polypropylene face stock. After coating anotherthinner gelatin layer (1-3 micrometers) on the backside, a pressuresensitive adhesive was coated over the thinner gelatin layer and thenlaminated to a high strength polyester liner.

Biaxially oriented polyolefin face stock

A composite sheet polyolefin sheet (31 μm thick) (d=0.68 g/cc)consisting of a microvoided and oriented polypropylene core(approximately 60% of the total sheet thickness), with a homopolymernon-microvoided oriented polypropylene layer on each side of the voidedlayer; the void initiating material used was poly(butyleneterephthalate). The polyolefin sheet had a skin layer consisting ofpolyethylene and a blue pigment. The polypropylene layer adjacent thevoided layer contained 4% rutile TiO₂ and optical brightener. The 7.5micrometer thick gelatin layers was applied to the blue tintedpolyethylene skin layer.

Pressure sensitive adhesive

Permanent solvent based acrylic adhesive (Gelva 2495, obtained as a 44percent solution from Solutia Inc.) 14 μm thick.

Polyester liner

A polyethylene terephthalate liner 37 μm thick that was transparent. Thepolyethylene terephthalate base had a stiffness of 15 millinewtons inthe machine direction and 20 millinewtons in the cross direction.Structure of the photographic packaging label material prior to addingthe image layer of the example is as follows:

Voided polypropylene base Acrylic pressure sensitive adhesive Polyesterliner

Label Test

The above prototype packaging label materials were hand applied toseveral flat untreated HDPE bottles to simulate application of the labelto a package. The bottles were placed in a controlled humidity oven at120° F. and 10% RH for 24 hours and the extent of curl induced labellift-off from the bottle of was determined by measuring the height ofthe highest point of the label from the surface of the bottle andcompared to a label with no gelatin coating on the backside (adhesiveside).

Table 1 list the variations of gelatin coatings that were coated on thebackside, underneath the adhesive layer of the prototype label to enablethe creation of a balanced architecture with regard to label curl. Thegelatin layers on both sides of the label were hardened withbis(vinylsulfonyl methyl) ether at 1.9 weight % of the total gelatinweight on each side. The effect of these layers in reducing label-curlwas evaluated at 120° F. 10% RH as described in the label test.

TABLE 1 Gelatin on Label Lift-off Sample # backside (g/m²) Adhesive(g/m²) (millimeters) 1 (Check) 0 15.9 5 2 1.07 15.9 0 3 2.15 15.9 0 43.23 15.9 0

Table 1 shows, the advantage of the balanced architecture. Althoughcoated underneath a very thick hydrophobic adhesive layer, in examples2-4, the curl under low humidities and high temperatures was eliminatedcompared to the check. The result is unexpected in light of the factthat in none of the cases was the gelatin on the backside exposed to theatmosphere.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

What is claimed is:
 1. An image element comprising at least one imagelayer, a base, a gelatin layer below said base and a pressure sensitiveadhesive below said gelatin layer, wherein said base has a stiffness ofless than 20 mN.
 2. The image element of claim 1 wherein said at leastone image layer comprises at least one hydrophilic colloid containinglayer.
 3. The image element of claim 2 wherein said at least onehydrophilic colloid containing layer further comprises an image formedutilizing dye forming couplers.
 4. The image element of claim 2 whereinsaid at least one hydrophilic colloid containing layer comprisesgelatin.
 5. The image element of claim 2 wherein said at least onehydrophilic colloid containing layer further comprises an image formedutilizing ink jet printing.
 6. The image element of claim 4 wherein saidat least one layer comprising gelatin has a thickness of greater than 3micrometers.
 7. The image element of claim 4 wherein said at least onelayer comprising gelatin has a thickness between 3 and 25 micrometers.8. The image element of claim 1 wherein said base has a thickness ofless than 100 micrometers.
 9. The image element of claim 1 wherein saidbase has a thickness of between 20 and 75 micrometers.
 10. The imageelement of claim 1 wherein said base has a stiffness of between 5 and 8millinewtons.
 11. The image element of claim 1 wherein said basecomprises a polymer sheet.
 12. The image element of claim 1 wherein saidgelatin layer below said base has a thickness of between 0.1 and 10microns.
 13. The image element of claim 1 wherein said gelatin layerbelow said base has a thickness of between 0.5 and 5 microns.
 14. Theimage element of claim 1 wherein said gelatin layer below said base hasa modulus of greater than 4000 MPa.
 15. The image element of claim 1wherein said gelatin layer below said base has a modulus of between 4000and 6500 MPa.
 16. The image element of claim 1 wherein said pressuresensitive adhesive comprises an acrylic adhesive.
 17. The image elementof claim 1 wherein said pressure sensitive adhesive comprises a urethaneadhesive.
 18. The imnage element of claim 4 wherein said element over arange of humidity of between 5 and 50% has a curl of less than 5 curlunits.
 19. The image element of claim 1 wherein said pressure sensitiveadhesive comprises gelatin in an amount of between 20 and 40% by weight.20. The image element of claim 1 wherein said bases comprises a nacreouspolymer base.
 21. The image element of claim 1 wherein said pressuresensitive adhesive further comprises between 4 and 12% by weight ofwhite pigment.
 22. The image element of claim 1 wherein said pressuresensitive adhesive layer is adhered to a carrier sheet.
 23. The imageelement of claim 22 wherein silicone is between said adhesive layer andsaid carrier sheet.